The invention relates to a roller according to the introductory part of claim 1. Rollers of this kind have a diameter of 0.3 to 1.50 meters and a length of up to 9 meters; the shell of the roller in the case of press rollers consists usually of granite, and, in the case of hard calender rollers, it consists of special cast iron with a sometimes chromium plated surface. As it is known, granite is an igneous rock which has a granular, speckled appearance, and consists usually of tightly packed crystals of quartz, feldspar, mica and a number of smaller components such as magnesite and iron pyrites which can be seen only with a microscope. The following material characteristics are found in the literature on granite:
______________________________________ Important Material Characteristics: ______________________________________ Tensile strength 10-20 N/mm.sup.2 Compressive strength 160-240 N/mm.sup.2 Compressive fatigue limit approx. 110 N/mm.sup.2 Modulus of elasticity 15,000-71,000 N/mm.sup.2 Poisson's number 0.2 Density 2.62-2.85 g/m.sup.2 Specific heat capacity 710-840 J/kgK Thermal conductivity 2.9-7.5 W/mK Thermal change of length 5-9 m/mK ______________________________________
In recent times it has emerged that the most critical of these material characteristics is the tensile strength, which is very low in all rock. Also, the tensile strength test is very complicated, since the shape, size and loading method have a great influence on the results and also, when considering its properties, it must be taken into consideration that granite, as a natural product, varies greatly, and local flaws in the structure can never be ruled out. With increasing size of the roller diameter and increasing speed of paper machines, the centrifugal force involved approaches the limit, and the first destructions of granite rollers have already been published, which not only resulted in severe disturbances of production but also constituted great danger to personnel in the vicinity of the machine.
To avoid the known disadvantages it has already been proposed to use press rolls whose outer periphery consists of plastic admixed with pulverized rock as filler--EP Patent 53 804.
It is likewise known to form the shell of the roller form a mixture of metal powder and a powder of inorganic substances, using as metals steel, nickel, chromium, titanium, copper or bronze, and, as the inorganic substances, quartz, feldspar, aluminum oxide, zirconium oxide, titanium oxide, titanium carbide, silicon carbide, magnesium oxide, silicon nitride, chromium oxide, tungsten carbide, niobium carbide, vanadium carbide, chromium carbide, or mixtures thereof--German Patent Disclosure Document 35 46 343.
It has furthermore been proposed to build up the shell of the press roller in the form of a hardened body of a hydraulic composition, i.e., from cements and mineral materials, and to cure it so that a compressive strength is achieved of not less than 1,000 kp/cm.sup.2 and a density of not less than 2.5. Portland cements, oil cements, silica cements and fly ash cements as well as blast-furnace cements have been proposed as the cements, and, as mineral materials, alite (CA.sub.3 SiO.sub.5), blast furnace slag, fly ash, and particles of crushed rock, such as granite, quartz, and feldspar, and additives, activators etc. are added in addition to water--German Patent Disclosure Document 36 17 316.
It is furthermore known to apply a coating to a shell of the roller made of metal, the coating consisting of chromium or stainless steel to which one or more components of a ceramic substance have been added. Al.sub.2 O.sub.3, SiO.sub.2, TiO.sub.2, ZrO.sub.2, SiC,TiC, Cr.sub.3 C.sub.2, WC, NbC, VC, and Cr.sub.7 C.sub.3 have been proposed as ceramic additive materials.
An additional proposal for the production of a press roll is contained in European Patent Disclosure Document 207,921. According to this proposal, first a metal intermediate layer of a lower coefficient of expansion than that of the roller body is applied to a metal roller, and onto that a porous ceramic is applied, both of the processes of application being based on plasma coating or flame spraying. The metal intermediate layer is applied so as to achieve the lowest possible difference between the coefficients of expansion of ceramic and metal.
In practice, however, all of these proposals have disadvantages which permit the use of such rollers not at all or to only a limited extent. For example, placing a metallic intermediate layer between ceramic coating and the supporting body does slightly reduce the effect of thermal expansion, but in the case of diameters of 1.50 m and over, and roller lengths of over 8 m, as are common in modern paper machines, the differences in expansion have been such that the bond between ceramic and metal is broken, leading to the destruction of the roller. With regard to common roller diameters, common lengths and temperature differences of up to 50 degrees, the result is the following when the machine is started up: with respect to the diameter, the metal core expands more than 600.mu. more than the ceramic shell, and with respect to the length the difference in expansion is about 3,500.mu..
Similar difficulties are encountered if a coating is applied which consists of plastics, and in some case also of metals and ceramic powders mixed therewith. In these cases, the complete coating does not come off as a layer, but the bond between the inorganic powder and the support layer does break down, resulting in increased wear.
The so-called synthetic stone rollers, which are virtually rollers of concrete, come closest to the granite natural product, but they also have their disadvantages, i.e., the surface is not uniform as regards grain distribution and granular composition, since here again natural products are used. The tensile strength, which is decidedly low though the compressive strength is very good, is just as critical as in the case of granite. The danger that a synthetic stone might fly apart at high speeds and large diameters, such as has occurred in the meantime in granite rock, is thus not eliminated.
Calender rollers of special cast iron present no difficulties as regards optimum strength, but in the use of these rollers the critical point is the surface of the roller, as regards hardness and average roughness R.sub.a, especially when papers containing abrasive fillers or paint pigments are to be calendered.
The problem to which the present invention is addressed is thus to retain the good surface properties which granite rock has in wet press rolling--namely, good release of paper from the roller, good wettability with water, less wear from scrapers--and nevertheless largely to eliminate the disadvantages of the natural product, especially to increase tensile strength, permit a uniform, repeatable surface structure, and become independent of granite quarries.
Another problem is to make calender rollers available with an improved surface for glazing paper.
In a roller of this general kind, this problem is solved by the specific part of claim 1 and the claims subordinate thereto.